According to the process requirements, choose half water gypsum, calcined gypsum used as a adhesive, Its content, particle size and crystal form must be determined according to the curing time, air permeability and other requirements. Hydration reaction using gypsum plays a role in bonding.
CaSo4 · 1/2H2O +3/2H2O = CaSO4 · 2H2O
In non-ferrous metal casting, Cast shells such as gypsum as a binder, Its quality score can reach 20% -40%. For dental molds, add 30%-50% hemihydrate gypsum as a binder. Since hemihydrate gypsum is divided into α-type hemihydrate gypsum and β-type hemihydrate gypsum, The strength of the short columnar α-type hemihydrate gypsum is greater than that of the small acicular cylinder. Therefore, α-type hemihydrate gypsum is used as a binder. In order to obtain hemihydrate gypsum or completely water-lost gypsum, calcining method is generally adopted, and calcined gypsum is directly used as a binder for convenience of use.
The most studied in China is plaster cast powder, For example, the gypsum cast powder studied by subetti was added 20% to 24% of the semi-hydrated gypsum, with the content (quality score) 28% to 38% of the alpha gypsum powder as the binder. Dai Duanwei added 15%-25% of hemihydrate gypsum.
2.1 Insoluble refractory powder
According to the mutual solubility of the powder, the different expansion coefficient, the crystal type transformation, the air permeability and the collapsibility of the interaction are selected.
Such as quartz, such as quartz powder and cristobalite, quartz powder content (mass fraction) can reach more than 90%, cristobalite content of up to 12.5% -29.2%
3,.1 other additives
(1) Time control agents, including accelerators and retarders.
As a coagulant, there are sulfates such as aluminum sulfate, magnesium sulfate, potassium sulfate, etc；Chloride salts such as sodium chloride, magnesium chloride, potassium chloride, etc；Nitrates such as KNO3，Ca(No3)2， LiNO3，Fe(NO3)2，NaNO3，Ni(NO3)2，Cu(NO3)2，Al(NO3)3，(NH4)NO3，Mg(NO3)2；Soluble organic acids such as alkyl benzene sulphonate or alkyl sulphate；Ammonium organic or inorganic salts
Retarders are polyanions such as fruit acids, maleic acid, maleates, oxalic acid, fatty acids, malonic acid, hydroxysuccinic acid, succinic acid oxalic acid, boric acid, phosphoric acid, citric acid, sodium citrate, lemon Trisodium acid, tartaric acid, sulfuric acid, acetic acid, formic acid, ascorbic acid, vitamin C, aspartic acid, its content (mass fraction) of 0.2% -18%, and sodium fluorosilicate.
（2）Dispersants, due to the phenomenon of agglomeration between the powder, and the viscosity of the slurry will also affect its overall performance, so it is necessary to add a dispersant, and its many types, such as: phosphates, alcohols, silanes and so on.
（3）Anti-foaming agent, this is because the bubbles in the slurry will affect the shell making operation and the quality of the shell. There are many types of anti-foaming agents and there are many divisions. In the selection process, the defoamer and other reagents should be considered in the coordination, the defoaming dose can account for 0-5%
（4）Wetting agents, which range from hydrophobic fatty acids such as oleic acid, linolenic acid. Linoleic acid. 2-ethyl valeric acid, etc.
（5）Sintering performance improver, because some of the properties of solid powder in the sintering process can not meet the requirements, some additives must be selected to improve. For example, when sintering ceramics, some manufacturers will add Y2O3, adding a small amount of rare earth can get good sintering and breaking properties.